Referring to FIG. 1, a known III-nitride heterojunction type power semiconductor device includes substrate 10, a III-nitride (e.g. AlN) buffer layer 12 formed over substrate 10, and III-nitride heterojunction 14 formed over buffer layer 12. III-nitride heterojunction 14 includes a first III-nitride body (e.g. GaN) 16 of one bandgap, and a second III-nitride body (e.g. AlGaN) 18 of another bandgap formed over first III-nitride body 16. The composition of first and second III-nitride bodies 16, 18 are selected to cause the formation of a carrier rich region referred to as a two-dimensional electron gas (2-DEG) at or near the heterojunction of the two bodies. The 2-DEG serves as the conduction channel between a first power electrode (e.g. source electrode) 20, and second power electrode (e.g. drain electrode) 22. Note that, typically, each power electrode 20, 22 is ohmically coupled to second III-nitride body 18 and thus electrically coupled through the same to the 2-DEG.
A typical device may further include a passivation body 24 made of a dielectric or the like formed over second III-nitride body 18, and a gate 26 disposed between first and second power electrodes 20, 33 to selectively interrupt/restore the 2-DEG therebetween, whereby the device may be operated as a switch. Note that gate 26 may include a gate dielectric body 30, and gate electrode 32.
Power electrodes 20, 22 as well as gate 26 extend through passivation body 24 to heterojunction 14, i.e. to second III-nitride body 18. Specifically, each extends through a trench (well) inside passivation body 24. The trench/well in which gate 26 is received includes vertical sidewalls that form sharp corners 28 with the top surface of second III-nitride body 18, creating high field regions at the bottom corners of gate 26 which reduce the breakdown voltage of the device. Further, vertical sidewalls that meet at second III-nitride body 18 increase the overlap between the gate electrode and the 2-DEG causing a high Qgd.
It would be desirable to reduce Qgd and increase the breakdown voltage of a III-nitride semiconductor device.